First we need two critters, close relatives, who have very different top speeds. Lions and cheetahs are perfect. They’re both big cats and they have the same basic design in leg joints.

Cheetahs are way faster. Cheetahs hit nearly 70 mph in a sprint. Lions can’t go much faster than 40 mph.

LONG ANKLES. Check out these two diagrams. Ok – where does the cheetah get its extra velocity? From its ankles. Much of the high speed comes from longer ankle bones. The ankle length compared to the thigh length is a reliable speed index in close relatives.

That’s an old Rule that Darwin knew back in 1859.

T-REX versus DUCK-BILL

Duck-bills were the most common big veggie-saurs in the time of the T-rex. The question is, could a T-rex catch a duck-bill?

We need to measure ankles compared to thighs in a rex and a duck-bill of the same geological time zone. Duck-bills and T-rexes have basically the same style of hind legs. And the hind legs deliver all or almost all of their forward thrust (Duck-bills did put their finger tips down on the ground while walking – but there wasn’t much weight on the forelimb.)

Check out diagram # 3. Here’s a T-rex with a thigh (femur) that’s 1200 mm long. And next to the T-rex is an Edmontosaurus, a big duck-bill.

Who has longer ankles?

THE T-REX!

The longest bones in the ankle are the metatarsals. And the rex has much longer metatarsals, compared to the duck-bill.

CONCLUSION:
T-REX COULD CATCH A DUCK-BILL EASILY, IN A SPRINT ON OPEN GROUND.

That’s cool. But now we have more questions – how fast was a duck-bill and a T-rex, in mph? And did a T-rex have the killing equipment necessary for bringing down live prey? Stay tuned for the answers to these questions.